Forming circuit



June 1, 1943. WATERMAN 2,320,495

FORMING CIRCUIT Filed Oct. 27. 1939 INVENTOR Herbert Wider/m2 ATTORNEYSPatented June 1,

FORMING CIRCUIT Herbert Waterman, New Bedi'ord, Mass, assignor toAerovox Corporation, New Bedford, Mass a corporation of New YorkApplication October 27, 1939, Serial No. 301,637

3 Claims.

The present invention relates more especially to the electrolyticformation of insulating film upon a surface of film forming metal and isshown particularly applied to the art of forming electrolyticcondensers.

Where it is attempted to apply the forming current to electrolyticcondensers in short shocks with intervening long periods of rest for thepurpose of minimizing deformation by preventing excessiye rise oftemperature, there is a tendency nevertheless to deform during the restand cooling period, because of the high temperature produced during theformation shocks. Such practice also involves the need for costlysupervision and the need for extensive investment in forming equipment,due to the length of time that each condenser section must remain on therack before it can be removed and replaced.

An object of the invention is to provide a film forming method performedby the use of simple equipment and with minimum power consumption, bywhich the film is built up under automatic control, without the need forskilled supervision and at substantially the maximum rate compatiblewith continuous progress :olf formation and without the need forinterpolating periods of rest orof cooling to guard against overheating.

According to the invention, the forming potential is applied through,thecondenser section, connected in series with a multi-element thermionictube and a resistance, the latter determining the negative bias of thegrid.

In the accompanying drawing in which is shown one of various possibleembodiments of the several features of the invention, the figure showsdiagrammatically, a circuit for forming electrolytic condensers,illustratively dry condenser sections.

The forming current is derived from a con ventional source ofalternating current, which is connected through a step up transformer Ito the mains II and I2 between which a difference of potential say of500 volts is maintained. Be-

tween the mains there is a multi-clement thermionic tube '1 including aplate P, a cathode, de-

sirably in the form of a filament F and a grid sistance R may be linearor non-linear and may have a substantial or a smallpositive or anegative or even a zero temperature coefilcient. The resistance'R in oneembodiment is desirably made up of an incandescent filament lamp R. ofpositive temperature coefficient in series with an ohmic wire resistanceRb. The grid G is connected by a conduit 20 to the same main I! as isthe resistance R. Appropriate clips (not shown) serve removably toconnect in circuit, a condenser section C to be formed, an electrode i4being connected to the other main II and the other electrode ii to theplate P.

If desired. the condenser sections could be connected between the saidmain l2 and the resistance R, instead of in the relation shown in thedrawing, or anywhere else in the circuit across the mains H and i2.

Only two thermionic tubes T and associated resistances R are shown, eachwith a condenser section C connected up to be formed, but it will beunderstood that in practice a multiplicity of like forming circuits areconnected in parallel across the mains.

Desirably the equipment may include a voltmeter V connected to the mainI I and having a lead v for selective connection thereof across anysection being formed to determine the voltage drop thereacross. Likewisethe equipment may include a milliammeter A connected to the main l2 andhaving a lead a. for selective connection in shunt with the resistanceRb to determine the current flow through any forming circuit, withoutopening such circuit.

The mode of operation will now be briefly set forth. The condensersections C either with unformed anodes or with anodes preformed inaqueous 'solution are connected between the plate P of the associatedtube and the main ll. Initially the resistance of the condenser sectionwill be relatively low, the electrolyte affording good conductionbetween its electrodes. The current through the condenser is primarilylimited by the effective resistance of the tube. That resistance isdetermined by the voltage drop across the resistance R, the bias beingequal to the voltage drop across such resistance. In the specificembodiment shown the resistance is determined by the current flowingthrough element Ra. When the current through the circuit decreases, thegrid bias (which'is the voltage drop across the resistance R) decreases,thus decreasing the eifective resistance of the tube and increasing thevoltage across the condenser section,

As formation of the dielectric film on the anode of the section Cprogresses, its ohmic resistance increases; with consequent reduction inthe current through the series connected parts C, T and R. Thedifference of potential between the ends of resistance R howeverdecreases more than in proportion to the current decrease since Thelamp-R. having a positive temperature resistance co-eflicient, thereresults a large increase in resistance when the current is high and acorresponding decrease when it is low. Thereby the negative grid bias isaccentuated as compared with the effect of that bias when a resistanceof constant magnitude is'used.

Thus, when the forming current would otherwise be too high, theresultant high negative grid bias depresses it and when it wouldotherwise be low, the resultant low negative grid bias raises it.Therefore an effective and automatic control is brought about; which onthe one hand precludes over-heating and deformation of the film, and onthe other hand accelerates the otherwise retarded rate of filmformation, particularly at more advanced stages of formation where afaster rate is permissible than would occur, were the forming currentfrom a constant voltage source passed through merely an inert resistancein series with the condenser section. The operation proceedsautomatically and without intermis sion, progressively and withoutpartial deformation at any stage. There is great economy of electricpower, and no loss due to over-heatin or burning out of sections. Thecomplete formation of a condenser section of 500 volts is thus safelyaccomplished in less than two hours and for some units in as little asfifteen minutes according to the present method, where from six totwelve hoursor more are required in commercial practices heretofore incommon use. The capital investment for forming equipment for a givenoutput is reduced in like ratio. The completion of the formation of eachsection can readily be verified by measuring with voltmeter V thevoltage across. the same, and with milliammeter A, the leakage currentthrough the section.

While tubes or resistances of a wide variety of construction could beemployed to advantage in carrying out the method set forth, it isadequate for present purposes to use a three-element vacuum tube of Type45, the filament current of which is 1.5 amperes applied at a voltage of2.5, the applied voltage of which tube is 275 and its current 36milliamperes and the grid bias of which is minus 56 volts. The lamp Ra.may desirably be of 10 to 15 watts and the resistance Rb in seriestherewith of about 12 ohms.

Since the multi-elernent thermionic tube it self serves as a rectifier,raw alternating current may be utilized for the formation, as previouslyset forth. It is of course also possible to use direct current whereavailable, for forming condensers according" to the method and with thecircuit set forth.

The forming method set forth is useful not only for ageing condenserswith preformed anode foil, but also for the entire formation of foil incondenser units that have not been preformed. while most useful forforming wound sections of dry electrolytic condensers, it is alsoapplicable to the formationpf the anodes of wet electrolytic condensers.

Wherever in the claims the term mains is used without specific referenceto alternating current and without distinguishing positive fromnegative, the claim is intended to be generic to direct and alternatingcurrent operation. In the former case it will be understood of coursethat the plate of the thermionic tube would be so disposed in thecircuit that positive potential is applied to the plate.

The invention is not limited in application to the formation ofcondensers or like electrolytic cells but maybe utilized in otherelectrolytic forming operations. In all applications the metal to beformed is. of the film forming type, and it is in contact with anappropriate electrolyte during the formation. 7

As many changes could be made in the above method and system and manyapparently widely different embodiments of this invention could be madewithout departing from the scope of the claims, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. Means for forming electrolytic condensers, comprising a condensersection having electrodes and intervening electrolyte, a multielementthermionic tube and a resistance unit connected in series with theelectrodes.of the condenser section, said tube having its grid connectedwith the negative end of the resistance unit, thereby to apply to saidgrid, a bias controlled by the decreasing leakage current through theprogressively forming condenser.

2. An installation for forming electrolytic condensers comprising a pairof mains, a condenser section to be formed having one electrodeconnected to one main, a multi-element thermionic tube having the platethereof connected to the remaining electrode of. the condenser sectionto be formed, a resistance unit connected in series with the thermionictube to the other main ofthe circuit, said resistance including anelement with a substantial temperature resistance co-efiicient, saidtube having a grid connected to the negative end of the resistance unitfor application of a bias to such grid, controlled by the leakagecurrent through the progressively forming condenser.

3. The installation defined by claim 2 in which the resistance unitincludes a filament lamp and an ohmic resistance connected in seriestherewith.

. HERBERT WATERMAN.

